1. Field of the Invention
The present invention relates to a microwave oven, and more particularly to a AC/DC type microwave oven which can be used with AC/DC power sources.
2. Description of the Prior Art
Generally, a microwave oven is an apparatus for cooking food by using a microwave. The microwave oven is provided with a high voltage transformer and a magnetron. The high voltage transformer serves to step up a common voltage of about 220V/110V to a high voltage of about 2,000V.about.4,000V. The magnetron is driven by the high voltage and radiates microwaves of a desired frequency. The microwaves vibrate molecules of moisture contained within the food. Therefore, the food is cooked by the frictional heat generated by the vibration of the moisture molecules. Here, the high voltage transformer receives an AC voltage via an input part thereof, and steps up or down the AC input voltage proportional to a turn ratio of a primary winding and a secondary winding thereof. The AC voltage which is stepped up or down is fed to an output part of the transformer. Typically, the conventional microwave oven described above is designed to be driven by an AC power source.
FIG. 1 is a circuit diagram showing the conventional microwave oven using the AC power source. In FIG. 1, a reference numeral 10 denotes a high voltage transformer, 11 is a primary coil, 12 is a first secondary coil, and 13 is a second secondary coil.
The primary coil 11 is wound on the input part of the high voltage transformer 10. The first and second secondary coils 12 and 13 are wound on the output part of the high voltage transformer 10. The primary coil 11 is connected with an AC power source AC. SW1 is a power switch. The power switch SW1 is located on a connecting wire which is disposed between the primary coil 11 and the AC power source AC, and connects or disconnects the primary coil 11 with the AC power source AC. A high voltage condenser HVC, a high voltage diode HVD and a magnetron MGT are connected to the output part of the transformer 10. The first secondary coil 12 pre-heats the magnetron MGT, and the second secondary coil 13 steps up the voltage provided by the AC power source to a voltage of about 2,000V. The second secondary coil 13 is connected with the magnetron via the high voltage condenser HVC and the high voltage diode HVD. The high voltage condenser HVC and the high voltage diode HVD are a voltage doubler to further step up the voltage raised by the second secondary coil 13 to a voltage of about 4,000V. The magnetron MGT is driven by the voltage of 4,000V and radiates a microwave of 2,450 MHz.
The operation of the conventional microwave oven constructed as above will be described as follows: If a user turns on the power switch SW1, the AC voltage is supplied to the high voltage transformer 10 via the power switch SW1. In the high voltage transformer 10, the AC input voltage is fed to the primary coil 11 of the input part and then induced to the first and second secondary coils 12 and 13 of the output part. The first secondary coil 12 pre-heats the magnetron MGT, and the second secondary coil 13 steps up the AC input voltage fed to the input part of the primary coil 11 to about 2,000V. The AC output voltage of about 2,000V, which is raised by the second secondary coil 13, is doubled by the high voltage condenser HVC and the high voltage diode HVD, and is then applied to the magnetron MGT. Therefore, the magnetron MGT is driven by the AC output voltage of about 4,000V and radiates a microwave of 2,450 MHz. The food within a cooking chamber (not shown) is cooked by the microwaves radiated by the magnetron MGT.
However, since the conventional microwave oven is designed to be driven by the common power source of AC 220V/110V, there is a problem that the conventional microwave oven can not be used in the open-air or in a ship, an aircraft or any other vehicles.
To overcome the above problem, there is proposed another conventional microwave oven that, when using the microwave oven in a place where an AC power source is not available, an inverter employing a separate semiconductor device may be connected with the microwave oven so as to invert a DC power source into an AC power source, or the inverter is disposed in the microwave oven itself.
FIG. 2 is a circuit diagram of a conventional microwave oven, and FIG. 3 is a circuit diagram of the inverter employing a semiconductor device. In FIG. 2, the construction of the part of AC power source is the same as FIG. 1, and in the part of the DC power source, there are disposed the inverter 20 employing a semiconductor device and a power switch SW2. The inverter employing a semiconductor device inverts the DC power source into the AC power source, and drives a high voltage transformer 10. A first primary coil 11 and a second primary coil 14 are wound on an input part of the high voltage transformer 10. The first primary coil 11 receives the AC power source, and the second primary coil 14 receives the AC power source inverted by the inverter 20. Further, a first secondary coil 12 and a second secondary coil 13 are wound on an output part of the high voltage transformer 10 along with a high voltage condenser HVC, a high voltage diode HVD and a magnetron MGT.
As shown in FIG. 3, the inverter 20 employing the semiconductor device comprises a trigger circuit 1, a plurality of thyristors th1 and th2 and a condenser C1. The plurality of thyristors th1 and th2 are switched on or off by a switching operation of the trigger circuit 1, and a current in the second primary coil 14 of the high voltage transformer 10 is thus outputted in turn, thereby generating the AC power source having a desired voltage in the high voltage transformer 10.
However, in this type of AC/DC microwave oven provided with the inverter employing the semiconductor device, there is a problem. That is, since it is necessary to provide a plurality of expensive semiconductor devices for the inverter in order to output a desired high voltage for the magnetron, the manufacturing cost is increased.
In the above conventional AC/DC microwave oven, there is another problem that the life span of the battery which supplies the DC power source is short, since the attrition rate of the current by the semiconductor device is very high.
In the above conventional AC/DC microwave oven, there is another problem that, since the semiconductor device generates excessive heat, energy loss by the heat is increased.
In the above conventional AC/DC microwave oven, there is a further problem that, since the size of the cooling fins is increased to cool the semiconductor device, the size of the microwave oven has also to be increased.